1. Field of the Invention
The present invention relates to a process for producing paraformaldehyde of high formaldehyde content.
2. Description of the Related Art
Heretofore, there has been commercially available paraformaldehyde containing about 80% by weight of formaldehyde (hereinafter, "% by weight" is simply called "%"). However, paraformaldehyde of high formaldehyde content has been recently demanded as a starting material for phenolic resins, urea resins, melamine resin, polyacetal and the like. Paraformaldehyde containing 90-94% of formaldehyde is now produced and moreover, paraformaldehyde containing 95% or more of formaldehyde can be produced.
When paraformaldehyde is used as a starting material for the above mentioned products, the paraformaldehyde is used in the form of a highly concentrated formaldehyde by dissolving it in warm water, hot water or various solvents, and therefore, paraformaldehyde is required to be highly soluble in warm water, hot water and the like.
As a method for producing a paraformaldehyde of a high formaldehyde content (hereinafter referred to as "high concentration paraformaldehyde"), the following is known. That is, formalin containing about 37-50% of formaldehyde is subjected to a concentration treatment such as vacuum concentration to produce a highly concentrated aqueous solution of formaldehyde containing about 80% of formaldehyde, and the resulting highly concentrated aqueous solution of formaldehyde is cooled and solidified by means of a cooling apparatus and a pelletizing column to form paraformaldehyde containing about 78-83% of formaldehyde in the form of particles or flakes (hereinafter referred to as "conventional paraformaldehyde"). Then the resulting conventional paraformaldehyde is dried at a temperature not higher than the softening point of the conventional paraformaldehyde by means of hot air drying or far infrared ray drying [Japanese Patent Application Laid-open No. Hei 2-167244 (1990)].
However, when conventional paraformaldehyde is dried by means of far infrared ray drying to produce a high concentration paraformaldehyde, the solubility of the high concentration paraformaldehyde in warm water or hot water becomes gradually lower with the lapse of time. That is, a high concentration paraformaldehyde immediately after the production and that stored for a long period of time, e.g. 30 days or more, after the production exhibit a big difference in solubility in warm water or hot water (hereinafter called "warm water-solubility").
Such difference in warm water-solubility is problematic when a high concentration paraformaldehyde is practically used.
In the case where only far infrared ray drying is employed, it is difficult to uniformly dry a somewhat thick material or superposed materials since far infrared ray can not inherently penetrate into a deep portion of materials, i.e. the penetration depth is usually 1 mm or less.
As a result, the degrees of drying states at the surface and the inside of a layer are different from each other, and therefore, there are produced materials of high degree of polymerization and, in a manner similar to hot air drying, the solubility in warm water or hot water becomes poor.
Furthermore, the degree of absorption of far infrared rays by paraformaldehyde is so high that the of paraformaldehyde in the form of particles or flakes is partly melted, and the particles or flakes become attached to each other to become a mass and it is not possible to dry uniformly the inside of the mass resulting in fluctuations in the degree of polymerization.
When such high concentration paraformaldehyde having poor solubility in warm water or hot water and fluctuation in the degree of polymerization is used for producing phenolic resins, such drawbacks adversely affect the condensation reaction with phenols resulting in a lower quality of the phenolic resins thus produced.
A standard for determining the degree of solubility of paraformaldehyde in warm water and hot water is "warm water-solubility". The warm water-solubility is determined by adding paraformaldehyde to a deionized water adjusted to 80.degree. C. in such an amount that the content of formaldehyde in water is 20%, stirring the resulting mixture until the paraformaldehyde is completely dissolved, and measuring the time required to be completely dissolved. This time is the warm water-solubility. This time is correlated with the degree of polymerization of the paraformaldehyde, and the longer this time, the higher the degree of polymerization.
When the turbidity of the resulting solution containing paraformaldehyde is 50 or less in terms of standard turbidity, the paraformaldehyde is deemed to be completely dissolved in water.
Depending on the use of the paraformaldehyde, the solubility may be determined by using butanol or the like in place of warm water.
Warm water-solubility of paraformaldehyde becomes gradually poor with the lapse of time after the production of the paraformaldehyde, and the higher the concentration of formaldehyde in paraformaldehyde, the poorer the warm water-solubility becomes.
Warm water-solubility of conventional paraformaldehyde (containing about 80% formaldehyde) is about 1-5 min. immediately after the production thereof by cooling and solidifying procedures, but 5-15 min. after 24 hours from the production, 20-40 min. after 10 days from the production, and 30-60 min. after 30 days from the production, and when longer than 30 days, the time of warm water-solubility increases gradually.
When conventional paraformaldehyde is dried by using a far infrared ray heating apparatus, warm water-solubility of the resulting high concentration paraformaldehyde is about 1-5 min. immediately after drying, 5-15 min. after 24 hours from drying, about 10-20 min. after 10 days from drying, undesirably as long as about 40-100 min. after 30 days from drying, and after longer than 30 days from drying, the time increases gradually.